1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a free-space atmospheric laser communications error control circuit.
2. Background Information
Free-space atmospheric laser communication systems transmit and receive information by means of a light beam that propagates through the atmosphere. When used for air-to-air or air-to-ground communications, such systems pose a number of challenging problems.
One such problem is that over long distances, or over short distances through a turbulent atmosphere, the beam is absorbed, diffracted and refracted, causing such problems as scintillation in the receiver. What the receiver sees is a "twinkling", like light from a star. The beam can actually disappear and reappear in a millisecond. The disappearance of the beam is called a "dropout". Such dropouts can disrupt normal communications 20-30 times a second if not controlled. Dropouts can last from one to ten milliseconds. Another form of disruption is attenuation of the beam due to absorption in haze, mist, fog, snow or other weather phenomenon. During such conditions, it is not uncommon to have the signal go from full-amplitude to nothing every few seconds.
The effect on communications of this "channel property" takes two forms: data can be lost and/or data can be corrupted. This is a problem for interfacing a free-space atmospheric laser communication system with modern network systems, which are designed to operate with a fast, reliable, low-error transport media (the physical layer). Accordingly, it is important to try to minimize or eliminate dropouts and to attain very low bit-error-rates. The present invention addresses the problem.